In a wireless multiple access communication system, a base station is responsible for communicating with multiple users. In general, the condition and characteristics of the wireless communication channel between a user and the base station can vary quite a lot from one user to another. The reason is that while channel fading is an ubiquitous phenomenon occurring in most wireless channels, the nature of the fading process can vary widely. For example, users who are moving rapidly experience fast fading, which can be challenging for the transmitter to track. On the other hand, stationary or pedestrian users normally experience channels with very slow fading that can be tracked accurately by the transmitter using feedback from the receiver. As the transmission techniques for achieving optimum performance depend on the condition and characteristics of channels on which communication takes place, it may be infeasible to have a single technique that performs well for all channel scenarios.
Several advanced communication techniques have been proposed for state-of-the-art wireless communication systems, many of which use multiple antennas at the transmitter and sometimes at the receiver. Some communication techniques are optimized for situations where the transmitter has multiple antennas while the receiver is constrained to have a single antenna. Within this category, some techniques, such as the Alamouti scheme, are optimized for receivers that perceive rapidly fading channels that can be tracked at the receiver but not at the transmitter. The Alamouti scheme is described in S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” IEEE Journal on Selected Areas in Communication, vol. 16, pp. 1451-1458, October 1998. There is a whole family of techniques, generally referred to as MIMO (multiple-input, multiple-output) techniques that are applicable in situations where the transmitter as well as the receiver have multiple antennas and can form a matrix channel. Some of these techniques are described in 1) V. Tarokh, N. Seshadri and R. Calderbank, “Space-time codes for high data rate wireless communication: Performance criterion and code construction,” IEEE transactions on Information Theory, vol. 44, pp. 744-765, March 1998 and 2) A. Naguib, N. Seshadri and R. Calderbank, “Increasing data rate over wireless channels,” IEEE Signal Processing Magazine, May 2000. These can, in general, extend performance along two dimensions. They can either be used for providing additional diversity (diversity gain), or they can be used to increase the data rate by establishing parallel data streams between transmit and receive antennas (spatial multiplexing). In general, a given space-time coding technique offers some diversity gain and some spatial multiplexing gain.
While different transmission technologies may provide benefits to one set of users in a multi-user system, other technologies may be better suited for providing signals to another set of users in the system. Furthermore, which technology provides the best method for supplying information to a user may change over time, e.g., as the user moves from one location to another and/or a users rate of movement changes with time. Accordingly, there is a need for methods and apparatus for providing a mobile user the benefits of a particular technology at a point in time which best suits the mobile's needs, reception characteristics and/or other mobile related characteristics such as motion characteristics, at the particular point in time. In addition, in a multi-user system, in would be desirable to be able to provide different wireless terminals, e.g., mobile devices, in a cell